To switch back to Yahoo Classic, you need to install a small plugin that is able to tell the browser version that you are using. An example of such plugin is User Agent Switcher. Then, you can open your mail and log in with your details. Option of "Updating your browser" and "Return to your previous version of Yahoo mail" will appear. Find out more from http://atechguide.com/switch-back-to-yahoo-classic-mail/.
I was able to change back by using <tools> <internet options> <security> and disabled my active java scripting. This allowed me to go back to classic. Once I was in classic - I had to go back and reset my active scripting to enable. As this feature does some other handy things in Yahoo Mail Classic! Good Luck :~}